1. Field of the Invention
The present invention relates to an apparatus and method for measuring fluid characteristics. More particularly, a device, including an integral reflecting means to provide an internal reference signal, useful in measuring fluid characteristics such as density, compressibility, acoustic impedance and attenuation is disclosed. A device constructed in accordance with the present invention is particularly useful for the remote monitoring of these characteristics, and is advantageously incorporated into a drill collar or the like for downhole applications.
2. Description of the Background
The measurement of various fluid characteristics in static and dynamic environments is quite important in many industrial processes. Such characteristics as the density, compressibility, acoustic impedance and attenuation of a fluid may contain important information for the system operator. Of particular interest in many applications is the determination of fluid density. Typically, to make a density measurement of a fluid, an accurate volumetric or dimensional measurement is required. However, for rapid or remote monitoring and particularly in dynamic situations, a simplified approach is desired. A probe which may be inserted within a flowing stream to measure these fluid characteristics without requiring flow interruption or volumetric measurement is desired.
Ultrasonic devices have been developed which are capable of providing some information concerning fluids in some circumstances. For example, devices using ultrasonic signals to determine the fluid level in containers are disclosed in U.S. Pat. Nos. 3,357,246, 4,144,517 and 4,203,324.
Determination of various fluid characteristics, particularly the density of the drilling fluid, is important during rotary drilling operations to prevent blowouts. In these operations, a drilling fluid, commonly referred to as drilling mud, is normally pumped down the drill string, exited through the drill bit and returned to the surface through the annulus formed between the drill string and the well bore. As the bit passes into and through various fluid containing zones, these fluids will enter the borehole if the pressure in the zone is greater than the pressure in the borehole. Lightweight fluids, e.g., salt water, light hydrocarbons and particularly gases, entering the borehole, decrease the density of the drilling fluid. Because the total weight of fluid within the borehole is critical to maintaining sufficient pressure to prevent blowouts when high pressure zones are entered, the remote determination of the density of the drilling fluid is extremely important.
U.S. Pat. No. 3,776,032 discloses a method and apparatus for detecting the inflow of fluid into a well during rotary drilling. This apparatus comprises a pair of acoustical transducers for transmitting high frequency voltage energy into the drilling fluid near the drill bit. The difference in the density of the fluid within the drill string near the drill bit and of the fluid within the annulus is determined. This difference or relative density is transmitted to the surface to indicate the influx of material into the borehole having a density different from that of the initial drilling fluid. This technique, while providing relative information, does not provide an absolute measurement of the density, compressibility, viscosity, acoustic impedance or attenuation of the fluid in the borehole. While this device and method provides a means of determining relative densities, it does not provide a device, including an internal referencing means, for making absolute measurements.
The device and method of the present invention overcome many of the disadvantages of the prior systems and provide a device useful for making accurate, remote measurements of fluid characteristics in a static or dynamic environment. This device and method are particularly useful in making measurements of fluid characteristics, e.g., the density of the drilling mud, at a remote downhole location during drilling. The knowledge of these characteristics permits the drilling rig operator to responsibly alter the characteristics of the drilling mud when required to maintain the desired density and to prevent the development of potential blowout conditions.
The art has long sought a simple device having an internal reference for measuring density and other fluid characteristics at remote locations in static and dynamic operations.